1. Field of the Invention
The present invention relates to an electrolytic processing apparatus and an electrolytic processing method, and more particularly to an electrolytic processing apparatus and an electrolytic processing method useful for processing a conductive material in a surface of a substrate, such as a semiconductor wafer, or for removing impurities adhering to a surface of a substrate.
2. Description of the Related Art
In recent years, instead of using aluminum or aluminum alloys as a material for forming circuits on a substrate such as a semiconductor wafer, there is an eminent movement towards using copper (Cu) which has a low electric resistivity and high electromigration resistance. Copper interconnects are generally formed by filling copper into fine recesses formed in the surface of a substrate. There are known various techniques for forming such copper interconnects, including chemical vapor deposition (CVD), sputtering, and plating. According to any such technique, a copper film is formed in the substantially entire surface of a substrate, followed by removal of unnecessary copper by chemical mechanical polishing (CMP).
FIGS. 1A through 1C illustrate, in sequence of process steps, an example of forming such a substrate W having copper interconnects. As shown in FIG. 1A, an insulating film 2, such as an oxide film of SiO2 or a film of low-k material, is deposited on a conductive layer lain which semiconductor devices are formed, which is formed on a semiconductor base 1. Contact holes 3 and interconnect trenches 4 are formed in the insulating film 2 by the lithography/etching technique. Thereafter, a barrier layer 5 of TaN or the like is formed on the surface, and a seed layer 7 as an electric supply layer for electroplating is formed on the barrier layer 5.
Then, as shown in FIG. 1B, copper plating is performed onto the surface of the substrate W to fill the contact holes 3 and the trenches 4 with copper and, at the same time, deposit a copper film 6 on the insulating film 2. Thereafter, the copper film 6, the seed layer 7 and the barrier layer 5 on the insulating film 2 are removed by chemical mechanical polishing (CMP) so as to make the surface of the copper film 6 filled in the contact holes 3 and the interconnect trenches 4, and the surface of the insulating film 2 lie substantially on the same plane. Interconnects composed of the copper film 6 as shown in FIG. 1C are thus formed.
Components in various types of equipments have recently become finer and have required higher accuracy. As sub-micron manufacturing technology is becoming common, the properties of materials are more and more influenced by the processing method. Under these circumstances, with a conventional mechanical processing method in which a processing object in a workpiece is physically destroyed and removed from the workpiece by a tool, many defects may be produced, deteriorating the properties of the workpiece. Thus, it is increasingly important to perform processing without deteriorating the properties of the materials.
Some processing methods, such as chemical polishing, electrolytic processing, and electrolytic polishing, have been developed in order to solve this problem. In contrast with the conventional physical processing, these methods perform removal processing or the like through chemical dissolution reaction. Therefore, these methods do not suffer from defects, such as formation of a damaged layer and dislocation, due to plastic deformation, so that processing can be performed without deteriorating the properties of the materials.
An electrolytic processing method that utilizes a catalytic activity of an ion exchanger and performs processing in ultrapure water has been developed. As shown in FIG. 2, an ion exchanger 12a mounted on a processing electrode 14 and an ion exchanger 12b mounted on a feeding electrode 16 are allowed to be close to or into contact with the surface of a workpiece 10. A voltage is applied from a power source 17 to between the processing electrode 14 and the feeding electrode 16 while a processing fluid 18, such as ultrapure water, is supplied from a fluid supply section 19 to between the processing electrode 14, feeding electrode 16 and the workpiece 10, thereby carrying out removal processing of the surface layer of the workpiece 10.
According to this electrolytic processing, water molecules 20 in the fluid 18, such as ultrapure water, are dissociated by the ion exchangers 12a and 12b into hydroxide ions 22 and hydrogen ions 24. The hydroxide ions 22 thus produced, for example, are carried, by the electric field between the workpiece 10 and the processing electrode 14 and by the flow of the fluid 18, to the surface of the workpiece 10 facing the processing electrode 14, whereby the density of the hydroxide ions 22 in the vicinity of the workpiece 10 is increased, and the hydroxide ions 22 are reacted with the atoms 10a of the workpiece 10. The reaction product 26 produced by reaction is dissolved in the fluid 18 such as ultrapure water, and removed from the workpiece 10 by the flow of the liquid 18 along the surface of the workpiece 10.
Since the processing principle of electrolytic processing is a chemical interaction between ions and a workpiece, removal processing of the workpiece is possible even when the workpiece is not in contact with an electrode as a processing tool. Thus, electrolytic processing, unlike mechanical processing, has the advantage of not causing physical damage to a workpiece. On the other hand, however, since removal processing proceeds in a non-contact manner, it is generally difficult to control the processing so that the processing can be properly terminated at the end point of processing (herein refers to a point in time when a desired processing amount is reached). Especially when electrolytically processing a workpiece, as typified by a semiconductor substrate, in which a plurality of materials (e.g. copper and a seed layer), having different electric conductivities, become exposed on the processing surface with the progress of processing, there occurs a rapid change in the processing rate due to a change in the area (processing area) between two different materials. This may result in overprocessing at the end point of processing.
In view of this, it may be considered to control the application of electric current or voltage in response to a change in the processing area between different materials so as to suppress overprocessing at the end point of processing. Such a method, however, will necessitate fairly complicated control of the application of electric current or voltage.